In Code Division Multiple Access (CDMA) communication systems, multiple data channels are provided by spreading the data of individual users with unique spread codes. Traditionally, the elements that are communicated on the data channels are referred to as ‘symbols’ and the elements of the spread codes are referred to as ‘chips’. An example of such a CDMA signal is the Wideband Code Division Multiple Access (WCDMA) signal specified by the 3rd Generation Partnership Project (3GPP) standards organization. Other examples for mobile wireless networks are the CDMA2000 family of signals and the TD-SCDMA signals contained in these two alternative CDMA technologies. These standardized CDMA technologies provide third generation (3G) mobile voice/phone and internet/data service to a growing number of wireless subscribers/users around the world.
At the CDMA signal transmitter, a CDMA signal for multiple channel transmission can be created by summing different spread code channel signals. These individual code channel signals are created by modulating a selected spread code sequence with a symbol from an assigned user data channel. At the CDMA signal receiver, knowledge of the spread code used at the transmitter is required so that the receiver can extract the data/symbol of interest to the user. This code knowledge is provided to the user's receiver, for example, as part of the network-subscriber link acquisition procedure. Under ideal radio wave propagation conditions, the performance of the CDMA receiver for an individual user data channel does not benefit from knowing the spread codes that are simultaneously being used on channels that have been assigned to other users.
Under realistic, non-ideal radio wave propagation conditions and in the real world environment of multiple base station networks, the multiple user/multiple base station CDMA signals interfere with each other such that the performance of traditional, ‘assigned code only’ receivers, can be severely degraded. An example of an ‘assigned code only’ receiver is a code-matched, channel-matched filter receiver which is also known as the implementation of a ‘Rake’ receiver which is well known in the art. When used for the downlink receiver in a subscriber handset, the Rake receiver displays performance degradation with increasing levels of intracellular and/or intercellular interference.
Intracellular interference refers to the multiple user interference within a single-base-station cell that arises due to multiple propagation path (multipath) distortion of the radio signal. Multipath distortion causes the spread codes associated with multiple code channels to loose their mathematical property of orthogonality. This loss of spread code orthogonality due to multipath results in a performance degradation of the legacy ‘assigned code only’ Rake receiver. This performance degradation is sufficiently severe to make the use of the Rake receiver undesirable for 3G networks designed to provide mobile wireless broadband service to multiple users. An ‘equalizer receiver’ is based on a signal-estimation filter that approximately corrects the multipath distortion and approximately restores the orthogonality property of the multiple spread code signals that are contained in the received CDMA signal. An ‘assigned code only’ despreading operation will then provide a high performance detection of the symbols that are of interest to the user, even in propagation environments that contain significant multipath-derived intracellular interference.
Intercellular interference refers to the multiple base station interference that arises whenever the subscriber's received signal contains signals originating from two or more base stations transmitting on the same radio frequency. The topology and the frequency usage of the 3G CDMA networks results in intercellular interference being a significant factor in a large amount of the cell coverage area. Both the legacy Rake receiver and the equalizer receiver are sensitive to intercellular interference and incur significant performance degradation when it is present.
More complicated, ‘advanced receivers’, that can mitigate the effects of intercellular interference for real world multiple user/multiple base station CDMA networks, include techniques that address intercellular interference such as ‘interference suppression by means of projection’, for example U. Madhow and M. L. Honig, MMSE Interference Suppression for Direct-Sequence Spread-Spectrum CDMA, IEEE Transactions on Communications, Vol. 42, No. 12, pp. 3178-3188, December 1994, and techniques that perform interference cancellation where interfering signals are estimated and subtracted. For examples of the latter type of advanced receiver see A. Bastug and D. Slock, Interference Canceling Receivers with Global MMSE—Zero Forcing Structure and Local MMSE Operations, Proc. Asilomar Conf. on Signals, Systems & Computers, November 2003. Interference cancellation receivers can provide a higher level of performance than interference suppression techniques, but special care must be taken to keep their implementations computationally efficient.
One practical problem that arises in the implementation of interference cancellation receivers is the computational burden associated with the additional signal processing required to individually despread each of the multiple CDMA signals, perform symbol decisions on each active code channel within each interfering signal, respreading the symbol decisions and finally summing the respread signals for all active code channels within each interfering signal.
For convenience, the sequential combination of code despread, symbol decision, code respread and respread summation operations will be referred to here as a “Despread-Respread” operation. The code despread operation is a series-to-parallel data conversion while the respread summation operation is a parallel-to-series data conversion. The complete Despread-Respread operation is necessary to obtain estimates of the active-code, chip-rate data in the interfering CDMA signals as they appear at the transmitting base stations. These estimates can then be multiplied by the scrambling code of the interfering base station, rechannelized and subtracted off from the received signal to accomplish the desired interference cancellation.
Another benefit of the Despread-Respread operation is that it allows a higher performance estimation of the propagation channel impulse response (CIR) as discussed by S. F. A. Shah and A. U. H. Sheikh, in the paper “Downlink Channel Estimation for IMT-DS”, in vol. 2 of the 12th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, 2001. Improving the quality of the channel estimate improves receiver performance by improving the processing that compensates for the multipath channel distortion, e.g., improved channel equalization. The improved CIR estimate also improves the accuracy of the rechannelization used in the interference cancellation.
Given the utility and benefit of the Despread-Respread operation for interference cancellation and high performance channel estimation, a system and method that provides an efficient means of performing this operation is desired.